The present invention relates to a recording medium with colored picture information, in particular a check card or identity card, and to a method for producing such a recording medium.
At the current level of reproducing technology the colored representation of picture motifs is customary and generally expected. Advertising also operates chiefly with color contrasts. Recording media such as bank cards, telephone cards, credit cards, etc., cannot resist this modern trend either, i.e. they are chiefly provided with colored motifs, in particular when they serve as advertising vehicles as well as performing their primary function.
A number of possibilities are known for transferring color pictures to such recording media. Alongside traditional photographic methods, it has also proven useful recently to use so-called transfer methods whereby the colorants are transferred from thin color media onto or into the picture-carrying surface by thermodiffusion or thermosublimation. Color pictures can also be produced by ink-jet technology or thermal printers, to mention only a few possibilities.
The disadvantage shared by all these methods, however, is that the applied color pictures are insufficient from the point of view of antifalsification since they only adhere more or less firmly to the surface of a carrier material. This thin, superficial layer containing the colorants can be removed or altered either intentionally or unintentionally. In the case of identity cards, which are valid e.g. for two years or more, there is the danger of mechanical wear or fading due to environmental influences.
One possibility for transferring a color picture that is reasonable from the security viewpoint is to use migrating inks, as are known from German patent no. 27 50 984. The migration of inks is based on first printing special inks on a plastic layer and then activating them by heat to migrate into the volume of the plastic carrier. It has the advantage over other methods for producing color pictures that the inks exist beyond the surface area within the volume of the plastic layer and can thus not be easily removed. The penetration depth can be controlled at will by adding to the plastic a reactive crosslinkable component that can be activated at the desired penetration depth, e.g. by irradiation with UV light. The resulting crosslinking stops any further migration of the colorant.
This method requires a special coordination of the inks used with the plastic carrier used. If the penetration depth of the inks is high, which is desirable for security reasons, this impairs the resolution or sharpness of the picture information.
From the point of view of antifalsification at the same time as high resolution it has proven useful to use so-called laser pencils, whereby the information to be transferred is burned into the material of the recording medium by a focused laser beam. German patent no. 19 07 004 discloses an identity card with a card inlay made of paper and a transparent cover film whereby certain information is inscribed or burned directly into the card inlay by laser beam after the cover film has been laminated on.
German patent no. 31 51 407 discloses a multilayer identity card with a specially designed transparent cover film. Despite its transparency in the visible region of the spectrum the cover film is able to absorb the energy of the laser beam so that information can be inscribed directly into the cover film.
The advantage of using a laser pencil is primarily that the resulting markings do not exist superficially but arise in the depth of the recording material. The carrier material is thereby discolored from its surface down to its deeper layers. The discoloration take place by local burning or local sooting; it is irreversible. At the same time the focal spot of the laser can be limited by precise control so as to produce portraits with clear and even extremely fine details.
When laser pencils and customary card structures are used the picture information is generally rendered by black-and-white contrast or corresponding half-tones. It is normally impossible to produce colored motifs having equivalent security.
In the prior art one must thus distinguish between information that can be incorporated in a recording medium as a high-resolution black-and-white contrast and is largely resistant to falsification, and color pictures that are applied by conventional methods but can either be removed or altered without great effort or are sufficiently resistant to falsification but have a low resolution.
The invention is based on the problem of developing known recording media so that they can be provided with colored picture information that shows fine details, is virtually irreversible and can almost not be falsified. Despite this high resistance to falsification the recording medium should also be of simple construction and inexpensive to produce.